1. Field
A process for synthesizing nanocrystals is disclosed.
2. Description of the Related Art
Nanoparticles have drawn much attention due to the fact that unlike bulk materials, their physical characteristics (e.g., energy bandgap and melting point) may be controlled by changing the particle size. For example, a semiconductor nanocrystal (also known as a quantum dot) is a semiconductor material having a crystalline structure of a size of several nanometers. The semiconductor nanocrystal has a very small size so that it has a large surface area per unit volume and may exhibit a quantum confinement effect. Therefore, the semiconductor nanocrystal has different physicochemical characteristics than the bulk material. A quantum dot may absorb light from an excitation source to be excited to an excited state, and may emit energy corresponding to its energy bandgap. In the quantum dot, the energy bandgap may be adjusted by varying the size and/or the composition of the nanocrystal. In addition, the quantum dot may emit light with high color purity. Therefore, various applications of the semiconductor nanocrystal in a display element, an energy device, a bio-light emitting element, or the like have been researched.
A semiconductor nanocrystal (i.e., a quantum dot) may be synthesized by a vapor deposition method such as metal organic chemical vapor deposition (“MOCVD”) and molecular beam epitaxy (“MBE”), or by a wet chemical method of adding a precursor to an organic solvent to grow nanocrystals. In the wet chemical method, an organic material such as a dispersant, is coordinated to a surface of the semiconductor crystal during the crystal growth to control the crystal growth. Therefore, the nanocrystals produced by the wet chemical method usually have a more uniform size and shape than those produced by the vapor deposition method.
A semiconductor nanocrystal for use in a device desirably has high quantum efficiency and high stability. However, when the semiconductor nanocrystal (e.g., a nanocrystal core) has a surface coordinated with an organic ligand without any shell layer, it tends to have many defects and/or traps, a very low level of light emitting efficiency, and insufficient stability (for example, the organic ligand may be easily removed, and this may cause oxidation of the nanocrystal). Passivation of the nanocrystal core with an inorganic shell may enhance the stability of the core and increase the efficiency due to the exciton confinement. In this regard, the differences in the crystal structures of the core/shell forming materials and the bandgap thereof may change the passivation degree and the shell quality, resulting in a difference in the efficiency of core/shell structures. Therefore, improvement of the shell passivation process may lead to improvements in the efficiency of the nanocrystals. Thus, an urgent need for technologies which enhance quality and yields of the core/shell nanocrystals remains.